Cyanine dyes

ABSTRACT

The present invention provides organic dye compounds having their absorption maxima in a region ranging from the ultraviolet region to a relatively short wavelength visible region and uses thereof. The present invention provides specific monomethine cyanine dyes, light absorbents and optical recording media comprising the monomethine cyanine dyes, and a process for producing the monomethine cyanine dyes which comprises a step of reacting a quaternary ammonium salt of nitrogen atom-containing heterocyclic compound having a reactive methyl group with a quarternary ammonium salt of nitrogen atom-containing heterocyclic compound having an appropriate leaving group.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a divisional of Ser. No. 09/913,730,filed Aug. 17, 2001 which is the national stage under 35 U.S.C. 371 ofinternational application PCT/JP00/08297, filed Nov. 24, 2000 whichdesignated the United States, and which international application waspublished under PCT Article 21(2) in the English language, the entirecontents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to novel organic dye compounds, andparticularly to monomethine cyanine dyes which are sensitive to visiblelight of a relatively short wavelength.

BACKGROUND OF THE IVENTION

[0003] In a multimedia age, optical recording media such as compact discrecordable (CD-R, a write-once memory using compact disc); and digitalversatile disc (DVD-R, a write-once memory using digital video disc),have been highlighted. Optical recording media can be classified roughlyinto inorganic optical recording media which have recording layerscomposed of inorganic substances such as tellurium, selenium, rhodium,carbon, or carbon sulfide; and organic optical recording media whichhave recording layers composed of light absorbents containing organicdye compounds.

[0004] Among these optical recording media, organic media are usuallyprepared by dissolving a polymethine dye in an organic solvent such as2,2,3,3-tetrafluoro-1-propanol (abbreviated as “TFP” hereinafter),coating the solution onto the surface of a polycarbonate substrate,drying the solution to form a recording layer, and sequentiallyattaching closely a reflection layer made of a metal such as gold,silver or copper and a protective layer made of an ultraviolet rayhardening resin onto the surface of the recording layer. When comparedwith inorganic optical recording media, organic optical recording mediahave the drawback that their recording layers may be easily changed byexposure to light such as reading- and natural light, but have the meritthat they can be manufactured at a lower cost because their recordinglayers can be formed by preparing solutions of light absorbents anddirectly coating the solutions onto the surface of substrates. Also,organic optical recording media are now becoming the predominantlow-cost optical recording media because of the merits that they aremainly composed of organic substances so that they are substantiallyfree of corrosion even when contacted with moisture or sea water; andbecause information, which is stored in optical recording media in aprescribed format, can be read out using a commercialized reader usingthermal deformation type optical recording media, a kind of organicoptical recording media.

[0005] What is urgently required of organic optical recording media isto increase their recording capacity to suit this multimedia age. Theresearch for such an increment now eagerly continued in this field is toshorten the wavelength of 635-650 nm now used as a writing light to awavelength of 450 nm or less to increase the recording capacity per oneside to a level from 4.7 giga bytes (GB) to 15 GB or higher. The opticalrecording media with such an increased capacity can record six hours ofmoving images in quality equivalent to standard TV and just record twohours of moving images in quality equivalent to high-quality TV.However, most of the organic dye compounds now used in optical recordingmedia are not applicable to laser beams with a wavelength of 450 nm orless, and therefore such organic dye compounds could not fulfill theneed for high storage density required in many fields.

SUMMARY OF THE INVENTION

[0006] In view of the foregoing, the object of the present invention isto provide organic dye compounds which are sensitive to visible lighthaving a relatively short wavelength, and to provide uses thereof.

[0007] To attain the above object, the present inventors eagerly studiedand screened compounds. As a result, they found that specificmonomethine cyanine dyes (may be called “monomethine cyanine dyes”hereinafter), which are obtainable through a step of reacting aquaternary ammonium salt of a nitrogen atom-containing heterocycliccompound having an active methyl group with a quaternary ammonium saltof a nitrogen atom-containing compound having a leaving group, have anabsorption maximum in a relatively short-wavelength visible region, andwhich substantially absorbs visible light in such a visible region. Theyalso found that, among these monomethine cyanine dyes, those which havesensitivity to laser beams with a wavelength of 450 nm or less when in athin layer form can form minute pits on the recording surfaces at arelatively high density when irradiated by a laser beam at a wavelength450 nm or less. The present invention was made based on the creation ofthe novel monomethine organic dye compounds which are sensitive tovisible light having a relative short wavelength, and the discovery oftheir industrially useful characteristics.

BRIEF DESCRIPTION OF THE DRAWING

[0008]FIG. 1 shows visible absorption spectra of one of the monomethinecyanine dyes of the present invention when in solution form and in thinlayer form.

DETAILED DESCRIPTION OF THE INVENTION

[0009] The present invention relates to the monomethine cyanine dyesrepresented by the Formula 1.

[0010] Formula 1:

ø1-CH=ø2

[0011] In Formula 1, ø 1 and ø 2 are the same or different heterocyclicgroups represented by any one of Formulae 2 to 8 as resonancestructures.

[0012] Throughout Formulae 2 to 5, Z represents, for example, a mono orpolycyclic aromatic ring or heterocycle such as benzene, naphthalene,pyridine, quinoline, naphthylidine or quinoxaline ring, which may haveone or more substituents. Examples of such substituents are halogenssuch as fluorine, chlorine, bromine, and iodine; ether groups such asmethoxy, ethoxy, propoxy, isopropoxy, butoxy, tert-butoxy, pentyloxy,benzyloxy, phenoxy, o-tolyloxy, m-tolyloxy, and p-tolyloxy groups; estergroups such as methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl,phenoxycarbonyl, o-tolyloxycarbonyl, m-tolyloxycarbonyl,p-tolyloxycarbonyl, acetoxy, and benzoyloxy groups; aromatic hydrocarbongroups such as phenyl, o-tolyl, m-tolyl, p-tolyl, xylyl, mesityl,o-cumenyl, m-cumenyl, p-cumenyl, nitrophenyl, and biphenyl groups;alkylsulfonyl groups such as methylsulfonyl, ethylsulfonyl,propylsulfonyl, and butylsulfonyl groups; alkylamino sulfonyl groupssuch as methylaminosulfonyl, dipropylaminosulfonyl, ethylaminosulfonyl,diethylamino sulfonyl, propylaminosulfonyl, dipropylaminosulfonyl, andbutylaminosulfonyl groups; methylene dioxy group; nitro group; cyanogroup; sulfo group; and aliphatic hydrocarbon groups such as those whichhave 1 to 5 carbon atoms such as methyl, ethyl, propyl, isopropyl,isopropenyl, 1-propenyl, 2-propenyl, butyl, isobutyl, sec-butyl,tert-butyl, 1-butenyl, 1,3-butadienyl, pentyl, isopentyl, neopentyl, and2-pentenyl groups. When the substituents have hydrogen atoms, one ormore of the hydrogen atoms may be substituted, for example, withhalogens such as fluorine, chlorine, bromine, and iodine. In Formulae 2to 5, when Z does not exist, one or more of the substituents like thosein the above Z may be bound to the position of Z. In the case of usingthe monomethine cyanine dyes of the present invention in opticalrecording media which use laser beams with a wavelength of not longerthan 450 nm, and in the case of using different types of cyclic cores ofø 1 and ø 2 into a non-symmetric structure as a whole molecularstructure and of forming either or both of the cyclic cores into acondensed ring, such a condensed ring is preferably restricted to form abicyclic structure as a whole cyclic core structure, varying dependingon that ø 1 and ø 2 are which of Formulae 2 to 5.

[0013] Throughout Formulae 2 to 8, R₁ represents an aliphatichydrocarbon group and R₂ represents a hydrogen atom or the same ordifferent aliphatic hydrocarbon group as in R₁. These aliphatichydrocarbon groups may have one or more substituents. Examples of suchaliphatic hydrocarbon groups are those which have one to eight carbonatoms, usually, for example, methyl, ethyl, propyl, isopropyl,isopropenyl, 1-propenyl, 2-propenyl, 2-propynyl, butyl, isobutyl,sec-butyl, tert-butyl, 2-butenyl, 2-butynyl, 1,3-butadienyl, pentyl,isopentyl, neopentyl, tert-pentyl, 1-methylpentyl, 2-methylpentyl,2-pentenyl, 2-pentene-4-ynyl, hexyl, isohexyl, 5-methylhexyl, heptyl,and octyl groups. One or more of the hydrogen atoms of these aliphatichydrocarbon groups may be substituted with halogens such as fluorine,chlorine, bromine, and iodine; ether groups such as methoxy,trifluoromethoxy, ethoxy, propoxy, isopropoxy, butoxy, tert-butoxy,pentyloxy, benzyloxy, and phenoxy groups; ester groups such asmethoxycarbonyl, trifluoromethoxycarbonyl, ethoxycarbonyl,propoxycarbonyl, acetoxy, trifluoroacetoxy, and benzoyloxy groups;aliphatic hydrocarbon groups such as phenyl, o-tolyl, m-tolyl, p-tolyl,xylyl, mesityl, o-cumenyl, m-cumenyl, p-cumenyl, nitrophenyl, andbiphenyl groups; heterocyclic groups such as 2-pyridyl, piperidino,pyrolidino, piperidinyl, morpholino, and 2-quinolyl groups; and otherssuch as hydroxy, carboxy, sulfo, and sulfonic acid ester groups.

[0014] Throughout Formulae 2 to 8, X⁻ represents an appropriate anion,usually, one selected from inorganic acid anions such as fluoride,chloride, bromide, iodide fluoric acid, chloric acid, bromic acid, iodicacid, perchloric acid, phosphoric acid, phosphoric acid hexafluoride,antimony acid hexafluoride, tin acid hexafluoride, fluoroboric acid, andtetrafluoroborate ions; organic acid anions such as thiocyanic acid,benzenesulfonic acid, naphthalenesulfonic acid, naphthalenedisulfonicacid, benzenecarboxylic acid, alkylcarboxylic acid, alkylsulfonic acid,trihaloalkylsulfonic acid, nicotinic acid ions, andtetracyanoquinonedimethane ions; and metal complex anions such as thoseof azo, bisphenyldithiol, thiocatecholchelate, thiobisphenolatechelate,bisdiol-α-diketone, and their related compounds. In Formulae 2 and 8, R₁or R₂ has a negatively charged substituent, and X⁻ does not exist whenthe substituent forms an internal salt.

[0015] The present invention relates to the monomethine cyanine dyeswhich have the aforesaid structures and absorption maxima in arelatively short wavelength visible region. Examples of the monomethinecyanine dyes are, for example, those represented by Chemical Formulae 1to 48 which have a variety of uses in the fields which require compoundsthat absorb light in such a visible region. The monomethine cyanine dyeshave absorption maxima in a region ranging from an ultraviolet region toa relatively short wavelength visible region, usually, at wavelengths of500 nm or less, more particularly, about 350-450 nm. In particular,among these monomethine cyanine dyes, those which are sensitive to laserbeams with wavelengths of 450 nm or less when in a thin-layer form,preferably, those which substantially absorb such laser beams in longerwavelength regions with their absorption maxima can be quiteadvantageously used as materials for high-density optical recordingmedia such as DVD-Rs, which use laser beams with wavelengths of 450 nmor less as a reading light.

[0016] The monomethine cyanine dyes according to the present inventioncan be prepared by various methods. When production cost is important,the monomethine cyanine dyes can be advantageously prepared through astep of reacting a quaternary ammonium salt of a nitrogenatom-containing heterocyclic compound having a reactive methyl groupwith a quaternary ammonium salt of nitrogen atom-containing heterocycliccompound having an appropriate leaving group. According to this method,the monomethine cyanine dyes used in the present invention can beproduced in a satisfactory yield by either reacting the compounds,represented by Formula 9 having ø1 corresponding to Formula 1, with thecompounds represented by Formula 10 having ø2 corresponding to Formula1; or reacting the compounds, represented by Formula 11 having ø1corresponding to Formula 1, with the compounds represented by Formula 12having ø2 corresponding to Formula 1. In Formulae 10 and 11, Lrepresents an appropriate leaving group, usually, a mercapto group oralkylthio group such as methylthio, ethylthio, or propylthio group.

[0017] Formula 9:

ø1-CH₃

[0018] Formula 10:

ø2-L

[0019] Formula 11:

ø1-L

[0020] Formula 12:

ø2-CH₃

[0021] For example, adequate amounts (usually, about equimolar) of thecompounds represented by Formulae 9 and 10 or the compounds representedby Formula 11 and 12 are optionally dissolved in an appropriate solventand reacted by heat refluxing at ambient temperature or at a highertemperature under heating and stirring conditions, after being admixedwith an adequate amount of a basic compound(s) such as sodium hydroxide,sodium bicarbonate, potassium carbonate, sodium acetate, potassiumacetate, ammonia, triethylamine, pyridine, piperidine, pyrrolidine,morpholine, 1,8-diazabicyclo[5.4.0]-7-undecene, aniline,N,N-dimethylaniline, or N-diethylaniline; an acid compound such ashydrochloric acid, sulfuric acid, nitric acid, methane sulfonic acid,p-toluenesulfonic acid, acetic acid, anhydrous acetic acid, anhydrouspropionic acid, trifluoroacetic acid, or trifluorosulfonic acid; or aLewis acid compound such as aluminum chloride, zinc chloride, tintetrachloride, or titanium tetrachloride.

[0022] Examples of the solvent include hydrocarbons such as pentane,hexane, cyclohexane, octane, benzene, toluene, and xylene; halogencompounds such as carbon tetrachloride, chloroform, 1,2-dichloroethane,1,2-dibromoethane, trichloroethylene, tetrachloroethylene,chlorobenzene, bromobenzene, and α-dichlorobenzene; alcohols and phenolssuch as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol,isobutyl alcohol, isopentyl alcohol, cyclohexanol, ethylene glycol,propylene glycol, 2-methoxyethanol, 2-ethoxyethanol, phenol, benzylalcohol, cresol, diethylene glycol, triethylene glycol, glycerine;ethers such as diethyl ether, diisopropyl ether, tetrahydrofuran,tetrahydropyran, 1,4-dioxane, anisole, 1,2-dimethoxyethane, diethyleneglycol dimethyl ether, dicyclohexyl-18-crown-6, methyl carbitol, andethylcarbitol; ketones such as furfural, acetone, ethyl methyl ketone,and cyclohexanone; acids and derivatives thereof such as acetic acid,anhydrous acetic acid, trichloroacetic acid, trifluoroacetic acid,anhydrous propionic acid, ethyl acetate, butyl carbonate, ethylenecarbonate, propylene carbonate, formamide, N-methyl formamide,N,N-dimethylformamide, N-acetamide, N,N-dimethylacetamide,hexamethylphosphoric triamide, and trimethyl phosphate; nitrites such asacetonitrile, propionitrile, succinonitrile, and benzonitrile; nitrocompounds such as nitromethane and nitrobenzene; sulfur-atom-containingcompounds such as dimethylsulfoxide and sulfolane; and water, which allcan be used in an appropriate combination, if necessary.

[0023] In the case of using such solvents, the greater the volume ofsolvents the lower the reaction efficiency. On the contrary, the lowerthe volume of solvents, the more difficult the homogenous heating andstirring, become and undesirable side-reactions may easily occur. Thus,the solvents should preferably be used in an amount up to 100 times byweight of the material compounds, usually, in the range of 5-50 times.Depending on the types of the material compounds and the reactionconditions used, the reaction should preferably be terminated within 10hours, usually, within 0.5-5 hours. The reaction procedure can bemonitored by conventional methods such as thin-layer chromatography, gaschromatography, and high-performance liquid chromatography. Aftercompletion of the reaction, the intact reaction mixture, if necessary,can be subjected to conventional counter-ion exchange reaction to obtainthe monomethine cyanine dyes, having a desired counter ion, of thepresent invention. All the monomethine cyanine dyes represented byFormulae 1 to 48 can be easily obtained by the above methods. Everycompound represented by Formulae 9 to 12 can be obtained in accordancewith the conventional methods for preparing cyclic cores in the relatedcompounds.

[0024] The monomethine cyanine dyes thus obtained can be used in theform of an intact reaction mixture, however, prior to use, they arepurified by conventional methods used for purifying the relatedcompounds such as dissolution, extraction, separation, decantation,filtration, concentration, thin-layer chromatography, columnchromatography, gas chromatography, high-performance liquidchromatography, distillation, crystallization, and sublimation. Ifnecessary, these methods can be used in an appropriate combination. Foruse in optical recording media such as DVD-Rs and dye lasers, themonomethine cyanine dyes of the present invention should preferably bepurified by methods such as distillation, crystallization and/orsublimation, prior to use.

[0025] The light absorbents referred to in the present invention includethose in general which contain one or more of the monomethine cyaninedyes, have sensitivity to visible light of a relatively short wavelengthinherent to the monomethine cyanine dyes, and use the properties tosubstantially absorb the relatively short-wavelength visible light,independently of the composition and the physicochemical properties ofthe light absorbents. Accordingly, the light absorbents of the presentinvention may be those which consist of the monomethine cyanine dyes andoptionally one or more other ingredients depending on use. One of thefields in which the light absorbents can be advantageously used is ofoptical recording media, and, in such a field, the light absorbents canbe preferably used as materials for composing recording layers fororganic optical recording media, particularly, high-density opticalrecording media which use laser beams with wavelengths of 450 nm or lessas a writing light. When used in optical recording media, the lightabsorbents can be used, if necessary, together with one or moreconventional materials used in optical recording media, for example,light absorbents containing other organic dye compounds sensitive tovisible light, light-resistant improvers, binders, dispersing agents,flame retardants, lubricants, antistatic agents, surfactants, thermointerference agents, plasticizers, colorants, developers, andsolubilizers.

[0026] The light absorbents of the present invention for use in organicoptical recording media or organic ablation-type optical recording mediacan be prepared in accordance with the methods used for conventionaloptical recording media because they do not need any special treatmentand handling when used in optical recording media. For example, tocontrol the reflectance and the absorptance in recording layers, themonomethine cyanine dyes can be, if necessary, incorporated with one ormore other organic dye compounds sensitive to visible light and furtherone or more conventionally used light-resistant improvers, binders,dispersing agents, flame retardants, lubricants, antistatic agents,surfactants, thermal interference agents, and plasticizers. Theresulting mixtures are then dissolved in organic solvents, and thesolutions are homogeneously coated over either surface of substrates byspraying, soaking, roller coating, or rotary coating method; and driedto form thin layers as recording layers containing light absorbents,and, if necessary, followed by forming reflection layers to be closelyattached on the recording layers by means of vacuum deposition, chemicalvapor deposition, sputtering, or ion-planting method using metals suchas gold, silver, copper, platinum, aluminum, cobalt, tin, nickel, iron,and chromium or using commonly used materials for organic reflectionlayers to attain reflection efficiency, which makes it possible to readrecorded information, for example, of 20% or higher, preferably, 30% orhigher. Alternatively, to protect the recording layers from scratches,dust, stains, etc., coatings may be applied over the recording layerswith ultraviolet ray hardening resins or thermosetting resins whichcontain flame retardants, stabilizers or antistatic agents, and then thecoatings are hardened by irradiating light or heating to form protectivelayers attached closely over the reflection layers. Thereafter, ifnecessary, a pair of the above substrates with recording-, reflection-,and recording-layers are faced and attached together using, for example,adhesives or viscous sheets; or protective plates, which are made of thesame materials and shapes as the substrates, are attached to theprotective layers of the substrates.

[0027] Other organic dye compounds usable in combination with themonomethine cyanine dyes of the present invention are not specificallyrestricted as long as they are sensitive to visible light and capable ofcontrolling the reflectance or the absorptance of recording layers ofoptical recording media when used with the monomethine cyanine dyes.Examples of such organic dye compounds are polymethine dyes such ascyanine, merocyanine, oxonol, azulenium, squallilium, styryl, pyrylium,thiopyrylium, and phenanthrene dyes, which have either a monomethinechain that may have one or more substituents or a polymethine chain suchas di-, tri-, tetra-, penta-, hexa-, and hepta-methine-chains, whereinthe both ends of the monomethine chain or the polymethine chain bind thesame or different cyclic cores such as imidazoline, imidazole,benzimidazole, α-naphthoimidazole, β-naphthoimidazole, indole,isoindole, indolenine, isoindolenine, benzindolenine,pyridinoindolenine, oxazoline, oxazole, isoxazole, benzoxazole,pyridineoxazole, α-naphthoxazole, β-naphthoxazole, selenazoline,selenazole, benzoselenazole, α-naphthoselenazole, β-naphthoselenazole,thiazoline, thiazole, isothiazole, benzothiazole, α-naphthothiazole,β-naphthothiazole, tellulazoline, tellulazole, benzotellulazole,α-naphthotellulazole, β-naphthotellulazole, aquaridine, anthracene,isoquinoline, isopyrrole, imidaquinoxaline, indandione, indazole,indoline, oxadiazole, carbazole, xanthene, quinazoline, quinoxaline,quinoline, chroman, cyclohexanedione, cyclopentanedione, cinnoline,thiodiazole, thiooxazolidone, thiophene, thionaphthene, thiobarbituricacid, thiohydantoin, tetrazole, triazine, naphthalene, naphthyridine,piperazine, pyrazine, pyrazole, pyrazoline, pyrazolidine, pyrozolone,pyran, pyridine, pyridazine, pyrimidine, pyrylium, pyrrolidine,pyrroline, pyrrole, phenazine, phenanthridine, phenanthrene,phenanthroline, phthalazine, pteridine, furazan, furan, purine, benzene,benzoxazine, benzopyran, morpholine, and rhodanine rings, which may haveone or more substituents. In addition, the following organic dyecompounds can be exemplified; acridine, azaannulene, azo, azo metalcomplex, anthraquinone, indigo, indanthrene, oxazine, xanthene,dioxazine, thiazine, thioindigo, tetrapyraporphyradine,triphenylmethane, triphenothiazine, napthoquinone, pyromethene,phthalocyanine, benzoquinone, benzopyran, benzofuranone, porphyrin,rhodamine dyes, and their related compounds. Depending on use, the aboveorganic dye compounds can be used in an appropriate combination.Preferable organic dye compounds used in combination with themonomethine cyanine dyes of the present invention are those which haveabsorption maxima in a visible region, particularly, those atwavelengths of 400-850 nm, when in a thin-layer form. The organic dyecompounds as disclosed in Japanese Patent Application No. 343,211/99,titled “Styryl dyes” and Japanese Patent Application No. 355,176/99,titled “Optical absorbents and uses thereof”, both of which were appliedfor by the same applicant of the present invention, are most preferablyused.

[0028] The light-resistant improvers used in the present invention are,for example, nitroso compounds such as nitrosodiphenylamine,nitrosoaniline, nitrosophenol, and nitrosonaphthol; and metal complexessuch as those of dithiolate and formazan, for example,tetracyanoquinodimethane compounds, diimmonium salts, “NKX-1199”(bis[2′-chloro-3-methoxy-4-(2-methoxyethoxy)dithiobenzyl]nickel)produced by Hayashibara Biochemical Laboratories, Inc., Okayama, Japan,which all can be used in an appropriate combination, if necessary.Preferable light-resistant improvers are those as disclosed in JapanesePatent Application No. 163,036/99, titled “Formazan metal complexes”applied for by the same applicant as the present invention, whichcontain metal complexes of dithiolate and formazan, most preferably,those which contain metal complexes of metals such as nickel, zinc,cobalt, iron, copper, palladium, etc., and as ligands one or more of theformazan derivatives and their tautomers, which have a pyridine ring atC-5 in the formazan skeleton and have a pyridine or furan ring bound toC-3 of the formazan skeleton. The combination use of the light-resistantimprovers lowers the solubility of the monomethine cyanine dyes of thepresent invention in organic solvents and effectively inhibits theundesirable deterioration, fading, color change, and quality change ofthe monomethine cyanine dyes, which are inducible by the exposure ofreading and environmental lights, without spoiling the preferableoptical properties of the monomethine cyanine dyes. As for thecomposition ratio, 0.01-5 moles, preferably, 0.1-1 mole of alight-resistant improver(s) can be incorporated into one mole of thepresent monomethine cyanine dye(s) while increasing or decreasing theratio. The light-resistant improvers should not necessarily existindependently of the monomethine cyanine dyes of the present invention,and if necessary, the monomethine cyanine dyes can be formulated intosalts, complexes, or compounds by combining with commonly used organicmetal complex anions, which are capable of improving the lightresistance, such as those of azo, bisphenyldithiol, phenylbisdiol,thiocatecholchelate, thiobisphenolatechelate, or bisdithiol-α-diketone,which are disclosed in Japanese Patent Kokai Nos. 19,355/89, 139,034/93,323,478/97, 6,651/98, etc., by using appropriate spacers andcrosslinking agents such as alkoxides or cyanates of metal elements, forexample, titanium, zirconium, aluminum, etc., or complexes of thesemetal elements having carbonyl compounds or hydroxy compounds asligands.

[0029] The monomethine cyanine dyes of the present invention havesatisfactory solubility in organic solvents without substantiallycausing problems and do not substantially restrict the types of organicsolvents used for coating the light absorbents on substrates. Thus, inthe preparation of optical recording media according to the presentinvention, for example, TFP used frequently to prepare optical recordingmedia and the following organic solvents other than TFP can be selectedand used in an appropriate combination: For example, hydrocarbons suchas hexane, cyclohexane, methylcyclohexane, dimethylcyclohexane,ethylcyclohexane, isopropylcyclohexane, tert-butylcyclohexane, octane,cyclooctane, benzene, toluene, and xylene; halogenides such as carbontetrachloride, chloroform, 1,2-dichloroethane, 1,2-dibromoethane,trichloroethylene, tetrachloroethylene, chlorobenzene, bromobenzene, andα-dichlorobenzene; alcohols and phenols such as methanol, ethanol,propanol, isopropanol, 2,2,2-trifluoroethanol, butanol, 2-butanol,isobutanol, isopentanol, cyclohexanol, ethylene glycol, propyleneglycol, 2-methoxyethanol (methyl cellosolve), 2-ethoxyethanol (ethylcellosolve), phenol, benzyl alcohol, cresol, diethylene glycol,triethylene glycol, glycerine, and diacetone alcohol; ethers such asdiethyl ether, diisopropyl ether, tetrahydrofuran, tetrahydropyran,1,4-dioxane, anisole, 1,2-dimethoxyethane, cyclohexyl-18-crown-6, methylcarbitol, and ethylcarbitol; ketones such as furfural, acetone,1,3-diacetyl acetone, ethyl methyl ketone, and cyclohexanone; esterssuch as ethyl acetate, butyl acetate, ethylene carbonate, propylenecarbonate, and trimethyl phosphate; amides such as formamide, N-methylformamide, N,N-dimethylformamide, N-methylacetamide,N,N-dimethylacetamide, and hexamethylphosphoric triamide; nitriles suchas acetonitrile, propionitrile, succinonitrile, and benzonitrile; nitrocompounds such as nitromethane and nitrobenzene; amines such as ethylenediamine, pyridine, piperidine, morpholine, and N-methylpyrrolidone; andsulfer atom-containing compounds such as dimethylsulfoxide andsulfolane.

[0030] Particularly, since the monomethine cyanine dyes of the presentinvention have relatively high solubility in easily volatile organicsolvents such as TFP, methyl cellosolve, ethyl cellosolve, and diacetonealcohol, they are substantially free from dye crystallization whensequentially dissolved in the organic solvents, coated on substrates,and dried; and do not cause inconsistent thickness and surface of theformed recording layers. Most of the monomethine cyanine dyes of thepresent invention have satisfactory solubility in non-halogen solvents,for example, cellosolves such as methyl cellosolve and ethyl cellosolve;alcohols such as diacetone alcohol; and ketones such as ethyl methylketone and cyclohexanone. Accordingly, the above non-halogen solventshardly damage substrates or spoil the environment when used to dissolvethe present light absorbents for coating on substrates.

[0031] The substrates used in the present invention are not specificallyrestricted and usually processed by forming appropriate materials, forexample, into discs, 12 cm in diameter and 0.1-1.2 mm in thickness,using the methods such as compression molding, injection molding,compression-injection molding, photopolymerization method (2P method),thermosetting integral method, and lightsetting integral method.Depending on their final use, the discs thus obtained can be usedsingularly or plurally after appropriately attaching them together withadhesives or adhesive sheets, etc. In principal, any one of thematerials for the substrates can be used in the present invention aslong as they are substantially transparent and have a transmittance ofat least 80%, preferably, at least 90% at a wavelength ranging from 350nm to 800 nm. Examples of such materials are glasses, ceramics, andothers such as synthetic resins including polyacrylate, poly(methylmethacrylate), polycarbonate, polystyrene (styrene copolymer),polymethylpentene, polyester, polyolefin, polyimide, polyetherimide,polysulfone, polyethersulfone, polyarylate, polycarbonate/polystyrenealloy, polyestercarbonate, polyphthalatecarbonate,polycarbonateacrylate, non-crystalline polyolefin, methacrylatecopolymer, diallylcarbonatediethylene-glycol, epoxy resins, and phenolicresins, among which polycarbonate- and acrylic-resins are usually usedfrequently. In the case of using plastic substrates, concaves forexpressing synchronizing-signals and addresses of tracks and sectors areusually transferred to the internal circuit of the tracks during theirformation. The form of concaves are not specifically restricted andpreferably formed to give 0.3-0.8 μm in average wide and 50-150 nm inwidth.

[0032] Considering the viscosity, the light absorbents of the presentinvention are prepared into 0.5-5% (w/w) solutions in the above organicsolvents, and then uniformly coated over substrates to form a driedrecording layer with 10-1,000 nm, preferably, 50-300 nm in thickness.Prior to the coating of the solutions, preliminary layers can be formedover the substrates to protect them and improve the adhesion ability ofthe substrates, if necessary. Materials for the preliminary layers are,for example, high molecular weight substances such as ionomer resins,polyamide resins, vinyl resins, natural resins, silicons, and liquidrubbers. In the case of using binders, the following polymers can beused alone or in combination in a weight ratio of 0.01-10 times of thelight absorbent(s): Cellulose esters such as nitrocellulose, cellulosephosphate, cellulose sulfate, cellulose acetate, cellulose propionate,cellulose lactate, cellulose palmitate, and celluloseacetate/propionate; cellulose ethers such as methyl cellulose, ethylcellulose, propyl cellulose, and butyl cellulose; vinyl resins such aspolystyrene, poly(vinyl chloride), poly(vinyl acetate), poly(vinylacetal), poly(vinyl butyral), poly(vinyl formal), poly(vinyl alcohol),and poly(vinyl pyrrolidone); copolymer resins such as styrene-butadienecopolymers, styrene-acrylonitrile copolymers,styrene-butadiene-acrylonitrile copolymers, vinyl chloride-vinyl acetatecopolymers, and maleic anhydride copolymers; acrylic resins such aspoly(methyl methacrylate), poly(methyl acrylate), polyacrylate,polymethacrylate, polyacrylamide, and polyacrylonitrile; polyesters suchas poly(ethylene terephthalate); and polyolefins such as polyethylene,chlorinated polyethylene, and polypropylene.

[0033] Explaining the method for using the optical recording mediaaccording to the present invention, the high-density optical recordingmedia such as DVD-Rs according to the present invention can writeinformation at a relatively high density by using laser beams withwavelengths of 450 nm or less, particularly, 350-450 nm irradiated bysemiconductor lasers such as those of InN, GaN, InGaN, InAlGaN, InGaNAs,BlnN, InGaNP, InP, GaP, GaAsP, and SiC, which oscillate in a blue orblue-violet region; or other laser beams, which oscillate in a redregion, for example, distributed feed back lasers in which secondharmonic generating mechanisms are installed in AlGaAs semiconductorlaser elements. To read information, laser beams are used which havewavelengths similar to or slightly shorter or longer than those used forwriting information. As for the laser power for writing and readinginformation, in the optical recording media of the present invention, itis preferably set to a relatively high level which exceeds the thresholdof the energy required for forming pits when used for writinginformation, while it is preferably set to a relatively low level, i.e.,a level below the threshold, when used for reading the recordedinformation, although the laser power level varies depending on thetypes and ratios of other light-resistant improvers used in combinationwith the light absorbents of the present invention: Generally, the laserpower level can be controlled by increasing or decreasing to a powerlevel of over 5 mW, usually, 10-50 mW for writing; and to a power levelof 5 mW or lower, usually, 0.1-5 mW for reading the recordedinformation. The recorded information is read out by detecting thechanges of both the reflection light level and the transmission lightlevel in the pits and the pit-less parts on the recorded surface ofoptical recording media.

[0034] Accordingly, in the optical recording media according to thepresent invention, quite minute pits with a pit width of below 0.5μm/pit and a track pitch of below 0.74 μm, which are below the levels ofthe existing DVD-Rs, can be formed smoothly at a relatively high densityusing a laser element with an oscillation wavelength of 450 nm or less.For example, in the case of using a substrate, 12 cm in diameter, onecan prepare an extremely high density optical recording medium having arecording capacity far exceeding 4.7 GB per one side, i.e., a recordingcapacity for about two hours of information in the form of images andvoices in quality similar to that of high quality televisions, whichrecording capacity could not be easily attained by the existing DVD-Rs.

[0035] Since the optical recording media according to the presentinvention can record information in the form of characters, images,voices, and other digital information at a relatively high density, theyare advantageously useful as recording media for professional and familyuse to record/backup/keep documents, data, and computer software.Particular examples of the kinds of industries and the forms ofinformation, to which the optical recording media of the presentinvention can be applied, are as follows: Drawings of construction andengineering works, maps, ledgers of loads and rivers, aperture cards,architectural sketches, documents of disaster protection, wiringdiagrams, arrangement plans, information from newspapers and magazines,local information, reports of construction works, etc., which all relateto architecture and civil construction; blueprints, ingredient tables,prescriptions, product specifications, product price tables, partslists, maintenance information, case study files of accidents andproblems, manuals for claims, production schemes, technical documents,sketches, details, company house-made product files, technical reports,analysis reports, etc., which all relate to manufacturing; customerinformation, correspondents information, company information, contracts,information from newspapers and magazines, business reports, reports ofcompany credibility, records of stocks, etc., which all relate to sales;company information, records of stocks, statistical documents,information from newspapers and magazines, contracts, customer lists,documents of application/notification/licenses/authorization, businessreports, etc., which all relate to finance; information regardingproperties, sketches of construction, maps, local information,information from newspapers and magazines, contracts of leases, companyinformation, stock lists, traffic information, correspondentsinformation, etc., which all relate to real property andtransportations; diagrams of writings and piping arrangements, documentsof disaster protection, tables of operation manuals, documents ofinvestigations, technical reports, etc., which all relate to electricand gas supplies; patient files, files of patient clinical histories andcase studies, diagrams of medical care/institution relationships, etc.,which all relate to medical fields; texts, collections of questions,educational documents, statistical information, etc., which all relateto private and preparatory schools; scientific papers, records inacademic societies, monthly reports of research, research data,documentary records and indexes thereof, etc., which all relate touniversities, colleges, and research institutes; inspection data,literatures, patent publications, weather maps, analytical records ofdata, customer files, etc., which all relate to information; casestudies on laws; membership lists, history notes, records ofworks/products, competition data, data of meetings/congresses, etc.,which all relate to organizations/associations; sightseeing information,traffic information, etc., which all relate to sightseeing; indexes ofhomemade publications, information of newspapers and magazines, who'swho files, sport records, telop files, scripts for broadcastings, etc.,which all relate to mass communications and publishing; and maps,ledgers of roads and rivers, fingerprint files, resident cards,documents of application/notification/license/authorization, statisticaldocuments, public documents, etc., which all relate to governmentoffices. Particularly, the write-once type optical recording media ofthe present invention can be advantageously useful for storing recordsof patient files and official documents, which must not be deleted orrewritten intentionally, and also used as electronic libraries for artgalleries, libraries, museums, broadcasting stations, etc.

[0036] As a rather specific use, the optical recording media of thepresent invention can be used to prepare and edit compact discs, digitalvideo discs, laser discs, MDs (a mini disc as information recordingsystem using photomagnetic disc), CDVs (a laser disc using compactdisc), DATs (an information recording system using magnetic tape),CD-ROMs (a read-only memory using compact disc), DVD-ROMs (a read-onlymemory using digital video disc), DVD-RAMs (a writable and readablememory using digital video disc), digital photos, movies, videosoftware, audio software, computer graphics, publishing products,broadcasting programs, commercial messages, computer software, gamesoftware, etc.; and used as external program recording means forlarge-sized computers and car navigation systems.

[0037] Hereinbefore, the use of the light absorbents of the presentinvention in the field of optical recording media has been mainlyexplained with reference to their application examples to organicoptical recording media which use laser beams with wavelengths of 450 nmor less as a writing light. However, in the field of optical recordingmedia, the light absorbents of the present invention can beadvantageously used not only in high-density optical recording media butcommonly used in optical recording media such as CD-Rs and DVD-Rs asmaterials for controlling and calibrating the absorptance and thereflectance by combining them with one or more other organic dyecompounds which are sensitive to laser beams with wavelengths of 635-650nm or 775-795 nm. Even in the case of using organic optical recordingmedia which use laser beams with wavelengths of 450 nm or less as awriting light, pits can be indirectly formed, without directly formingpits on substrates using the monomethine cyanine dyes of the presentinvention, by combining with one or more other organic dye compoundssensitive to a longer wavelength light, for example, a laser beam with awavelength of 635-650 nm or 775-795 nm, in such a manner that an exitedenergy by laser beams with wavelengths of 450 nm or less is transferredthrough the monomethine cyanine dyes to the organic dye compounds todecompose the compounds. The term “optical recording media” as referredto in the present invention means those in general which use thecharacteristic features of specific monomethine cyanine dyes that haveabsorption maxima in a relatively short-wavelength visible region andsubstantially absorb such a visible light, and includes, in addition tothe organic optical recording media, for example, those prepared by thethermal coloration method which uses the chemical reaction of coloringagents and developers induced by the heat generated when the organic dyecompounds absorb light, and those prepared by the technique called“moth-eye type technique” which uses the phenomenon that the above heatsmoothes the pattern of periodical unevenness, provided on the surfaceof substrates.

[0038] The monomethine cyanine dyes of the present invention haveabsorption maxima in a region ranging from an ultraviolet region to arelatively short wavelength visible region and substantially absorblight in such a visible region, and therefore in addition to being usedin the aforesaid optical recording media, they can be advantageouslyused as materials for polymerizing polymerizable compounds by exposureto visible light, light absorption materials for lithography, laseraction substances in dye lasers which oscillate in a blue or blue-violetregion, and light absorbents for dying clothes. If necessary, incombination with one or more other light absorbents capable of absorbinglight in ultraviolet, visible and/or infrared regions, the lightabsorbents of the present invention can be used in clothes in generaland other materials including building/bedding/decorating products suchas drapes, laces, casements, prints, venetian blinds, roll screens,shutters, shop curtains, blankets, thick bedquilts including comforters,peripheral materials for thick bedquilts, covers for thick bedquilts,cottons for thick bedquilts, bed sheets, Japanese cushions, pillows,pillow covers, cushions, mats, carpets, sleeping bags, tents, interiorfinishes for cars, and window glasses including car window glasses;sanitary and health goods such as paper diapers, diaper covers,eyeglasses, monocles, and lorgnettes; internal basesheets/linings/materials for shoes; wrappers; materials for umbrellas;parasols; stuffed toys; lighting devices; filters/panels/screens forinformation displaying devices such as televisions and personalcomputers which use cathode-ray tubes, liquid crystal displays,electroluminescent displays, and plasma displays; sunglasses; sunroofs;sun visors; pet bottles; storage; vinyl houses; lawns; optical fibers;prepaid cards; and windows of ovens including electric ovens. When usedfor wrapping, injecting, and enclosing the above articles, the lightabsorbents of the present invention advantageously prevent living bodiesand products from problems and discomforts induced by environmentallights such as natural and artificial light or minimize the aboveproblems and discomforts. Furthermore, they can advantageously regulatethe color, tint, and appearance and adjust the light reflected from orpassed through the articles to a desired color balance.

[0039] The following examples describe the preferred embodimentsaccording to the present invention:

EXAMPLE 1 Monomethine Cyanine Dyes

[0040] Five grams of 3-ethyl-2-methylthiazolium iodide, six grams of3-methyl-2-methylthiobenzoxazolium methylsulfate, 2.5 ml oftriethylamine, and 25 ml of acetonitrile were placed in a reactionvessel, and the mixture was heat refluxed for two hours, followed byremoving the acetonitrile by distillation and washing the resultingresidues with ethyl ether and acetone. The crude crystal formed wascollected and recrystallized in ethanol to obtain 1.3 g of a yellowcrystal of the monomethine cyanine dye, represented by Chemical Formula4. Upon conventional measurement, the crystal had a melting point of199-200° C.

[0041] The monomethine cyanine dye with satisfactory optical propertiesthus obtained can be used in various fields as light absorbentsincluding optical recording media.

EXAMPLE 2 Monomethine Cyanine Dye

[0042] Three grams of 2,3,4-trimethyloxazolium iodide, 2.4 g of3-ethyl-4-methyl-2-methylthiothiazolium iodide, 2 ml of triethylamine,and 20 ml of acetonitrile were placed in a reaction vessel, and themixture was heat refluxed for two hours. Thereafter, the reactionmixture was treated similarly as in Example 1 to obtain 0.8 g of ayellow crystal of the monomethine cyanine dye represented by ChemicalFormula 17. Upon conventional measurement, the crystal had a meltingpoint of 308° C.

[0043] The monomethine cyanine dye with satisfactory optical propertiesthus obtained can be used in various fields as light absorbentsincluding optical recording media.

EXAMPLE 3 Monomethine Cyanine Dye

[0044] 2.8 g of 3-methyl-2-methylthiobenzoxazolium methylsulfate, 3 g of1,2,3,3-tetramethylindolenium iodide, 24 ml of pyridine, and 1.2 ml ofacetic acid were placed in a reaction vessel, and the mixture was heatrefluxed for three hours. The solvents were removed from the reactionmixture, and the residue was admixed with 20 ml of methanol andsubjected to counter-ion exchange by the addition of 4.8 ml aqueoussolution containing 1.6 g potassium iodide. Thereafter, the crudecrystal formed was collected and recrystallized from methanol to obtain1.2 g of a yellow crystal of the monomethine cyanine dye represented byChemical Formula 36. Upon conventional measurement, the crystal had amelting point of 267-269° C.

[0045] The monomethine cyanine dye with satisfactory optical propertiesthus obtained can be used in various fields as light absorbentsincluding optical recording media.

EXAMPLE 4 Monomethine Cyanine Dye

[0046] 3.7 g of 3-methyl-2-methyl-5-phenylbenzoxazolium iodide, 2.9 g of3-ethyl-2-methylthiobenzoxazolium iodide, 2.2 ml of triethylamine, and20 ml of acetonitrile were placed in a reaction vessel, and the mixturewas heat refluxed for two hours. The reaction mixture was treatedsimilarly as in Example 1 to obtain 2.5 g of a yellow crystal of themonomethine cyanine dye represented by Chemical Formula 6. Uponconventional measurement, the crystal had a melting point of 280° C.

[0047] Although the production conditions and the yields of themonomethine cyanine dyes used in the present invention are somewhatvaried depending on their structures, they, including the compoundsrepresented by Formulae 1 to 48, can be produced in a desired yield bythe methods in Examples 1 to 4 which comprise a step of reacting aquaternary ammonium salt of nitrogen atom-containing heterocycliccompound having a reactive methyl group with a quaternary ammonium saltof nitrogen atom-containing heterocyclic compound having an appropriateleaving group, or can be produced in accordance with conventionalmethods.

EXAMPLE 5 Optical Properties of Monomethine Cyanine Dye Example 5-1Light Absorption Properties of Monomethine Cyanine Dye

[0048] The monomethine cyanine dyes in Table 1 were measured for theirvisible absorption spectra when dissolved in methanol and formed onglass substrates, respectively. The results are tabulated in Table 1,and the visible absorption spectrum of the monomethine cyanine dyerepresented by Chemical Formula 17 when in a liquid form and in athin-layer form are shown in FIG. 1. TABLE 1 Wavelength of absorptionmaximum (nm) Monomethine cyanine dye Solution Thin layer ChemicalFormula 4 350 351 Chemical Formula 6 380 371 Chemical Formula 14 387 380Chemical Formula 16 383 374 Chemical Formula 17 379 375 Chemical Formula36 396 440 Chemical Formula 38 341 342

[0049] As evident from the results in Table 1 and FIG. 1, themonomethine cyanine dyes tested had absorption maxima in relativelyshort wavelength visible regions, particularly, at wavelengths of 450 nmor less both in the forms of solution and thin layer. Most of thesemonomethine cyanine dyes had absorption maxima at wavelengths of 350-400nm when in solution and thin layer forms, and the ends of theirabsorption maxima in their longer wavelength regions extended up toabout 450 nm when in a thin layer form. The fact evidences that thelight absorbents, comprising the monomethine cyanine dyes of the presentinvention, have sensitivity to a relatively short-wavelength visiblelight, and most of the light absorbents substantially absorb laser beamswith wavelengths of 450 nm or less in their longer wavelength regionswith their absorption maxima.

Example 5-2 Light-Resistance Improvement for Monomethine Cyanine Dye

[0050] Fifteen milligrams of either of the monomethine cyanine dyes inTable 2 was added to three milliliters of TFP, and as a light-resistantimprover two milligrams of the formazan nickel complex represented byChemical Formula 49 disclosed in Japanese Patent Application No.163,036/99, titled “Formazan metal complexes” applied for by the sameapplicant as the present invention, and the contents were dissolved inthe solvent with a 5-minute ultrasonic energization at ambienttemperature. Thereafter, in a usual manner, a prescribed volume of theresulting solution was dropped on either surface of a polished glasssubstrate, 5 cm×5 cm, while the glass substrate was rotated at arotation rate of 1,000 rpm for one minute to uniformly coat the solutionthereupon, and sequentially blown with hot air and cold air to dry thecoated solution.

[0051] The resulting glass substrates coated with the monomethinecyanine dyes were measured for transmittance (T₀) at the wavelengths oftheir absorption maxima, and then fixed to a position 7 cm apart from a500 W xenon lamp and exposed with the light for 25 min while cold airwas blowing to the substrates. Immediately after that, the resultingsubstrates were remeasured for transmittance (T) at the wavelengths ofthe absorption maxima of the monomethine cyanine dyes, and thetransmittances of T and T₀ for each monomethine cyanine dye weresubstituted for the Equation 1 to calculate the residual percentage (%)of each monomethine cyanine dye. In parallel, control systems with nolight-resistant improver for each monomethine cyanine dye were providedand treated similarly as above. The results are shown in Table 2.${{{{Equation}\quad 1}:\quad \begin{matrix}{{{Residual}\quad {percentage}\quad (\%)\quad {of}}\quad} \\{{{monomethine}\quad {cyanine}\quad {dye}}\quad}\end{matrix}} = {\frac{100 - T}{100 - T_{0}} \times 100}}\quad$

TABLE 2 Residual percentage (%) of monomethine cyanine dye MonomethineWith light-resistant With no light-resistant cyanine dyes improverimprover Chemical Formula 4 100.0 88.6 Chemical Formula 5 100.0 96.6Chemical Formula 6 100.0 98.7 Chemical Formula 17 100.0 80.6 ChemicalFormula 36 93.2 26.0

[0052] As shown in the results in Table 2, in the systems with nolight-resistant improver, up to 74% of the monomethine cyanine dye hadchanged with only a 25-min exposure of light to become incapable ofexerting its inherent optical properties. However, in the systems withthe formazan metal complex represented by Chemical Formula 49, all ofthe monomethine cyanine dyes still remained intact up to a level of over93% without changing even after the exposure of light. These resultsindicate that light-resistant improvers such as formazan metal complexesquite effectively inhibit the undesirable changing of the monomethinecyanine dyes inducible by the exposure of light such as natural- andartificial-lights.

EXAMPLE 6 Optical Recording Medium

[0053] To TFP was added, as a light absorbent, the monomethine cyaninedye, represented by Formula 6, 14, 16 or 17, to give a concentration of3.0 w/w %, and the mixture was mixed with, as a light-resistantimprover, the formazan metal complex represented by Chemical Formula 49to give a concentration of 0.35% (w/w), and then heated and energizedwith ultrasound to dissolve the contents. The solution was in a usualmanner filtered, and the filtrate was coated in a rotary manner over oneside of an acrylic disc substrate, 12 cm in diameter, which concaves forexpressing synchronizing signals and addresses of tracks and sectors hadbeen transferred to the track's internal circuit, and dried to form arecording layer, 200 nm in thickness. Thereafter, the substrate wasspattered with silver to form a reflection layer, 100 nm in thickness,to be closely attached on the surface of the recording layer, and thereflection layer was homogeneously coated in a rotatory manner with“DAICURE CLEAR SD1700”, as a known ultraviolet ray hardening resincommercialized by Dainippon Ink and Chemicals, Inc., Tokyo, Japan, andirradiated to form a protective layer to be closely attached on thesurface of the reflection layer. Thus, four types of optical recordingmedia were obtained.

[0054] Every optical recording medium in this example can write a largeamount of information in the form of documents, images, and voices at arelatively high density by using laser elements that oscillate atwavelengths of 450 nm or less.

[0055] As described above, the present invention was made based on thecreation of novel monomethine cyanine dyes and the discovery of theirindustrially useful properties. Since the light absorbents of thepresent invention have absorption maxima in the visible region of arelatively short wavelength and substantially absorb the light in such avisible region, they have a variety of uses in the fields, for example,of optical recording media, optical polymerization, dye lasers, solarbatteries, lithography, and dyeing. Particularly, the monomethinecyanine dyes, which substantially absorb visible light with wavelengthsof 450 nm or less when in a thin layer form, can be advantageouslyuseful as a light absorbent in high-density optical recording media suchas DVD-Rs.

[0056] The optical recording media of the present invention, whichcontain the aforesaid light absorbents and use as a writing light laserbeams with wavelengths of 450 nm or less, can form more minute pits on arestricted recording surface of the optical recording media at a shortertrack pitch and at a relatively high density, as compared with theDVD-Rs now available which use polymethine dyes as light absorbents andwrite information using laser beams with wavelengths of 635 nm or 650nm. Thus, with the optical recording media of the present invention,information in the form of characters, images, and voices, and otherdigital information can be recorded in a single disc of opticalrecording medium at a relatively high density and in a large amount. Asa result, the cost per a bit required for recording information can belowered by a large margin, and moving images and static images can berecorded for a relatively long period of time.

[0057] The monomethine cyanine dyes of the present invention with suchusefulness can be easily obtained in a desired yield through a step ofreacting a quaternary ammonium salt of nitrogen-atom-containingheterocyclic compound having a reactive methyl group with a quaternaryammonium salt of nitrogen atom-containing heterocyclic compound havingan appropriate leaving group.

What is claimed is:
 1. A light absorbent comprising a monomethinecyanine dye of Formula 1: Formula 1: ø1-CH=ø2 wherein in Formula 1, phi1 and phi 2 are the same or different heterocyclic groups represented byany one of Formulae 2 to 8 as resonance structures:

wherein throughout Formulae 2 to 5, Z represents an optionallysubstituted mono- or polycyclic aromatic ring or heterocycle whichcondenses to a nitrogen atom-containing five-membered heterocycle; whenZ does not exist in Formulae 2 to 5, a substituent similar to that for Zis bound to the position of Z; throughout Formulae 2 to 8, R₁ representsan aliphatic hydrocarbon group and R2 represent a hydrogen atom or thesame or different aliphatic hydrocarbon group as in R₁, wherein thealiphatic hydrocarbon groups may have a substituent, and in formula 2 to8, X− represents an appropriate anion.
 2. The light absorbent accordingto claim 1 wherein the anion is an organic metal complex anion capableof improving light resistance.
 3. The light absorbent according to claim1 or claim 2 which has an absorption maximum at a wavelength of 450 nmor less.
 4. The light absorbent according to claim 1, 2, or 3 whichsubstantially absorbs visible light with a wavelength of 450 nm or lesswhen in a thin layer form.
 5. The light absorbent according to claim 1which further includes at least one other organic dye compound sensitiveto visible light.
 6. The light absorbent according to claim 1 or 5 whichfurther includes at least one appropriate light-resistant improver. 7.The light absorbent according to claim 1, 5, or 6 which is sensitive tovisible light with a wavelength of 450 nm or less when in a thin layerform.
 8. An optical recording medium comprising a monomethine cyaninedye of Formula 1: Formula 1: ø1-CH=ø2 wherein in Formula 1, phi 1 andphi 2 are the same or different heterocyclic groups represented by anyone of Formulae 2 to 8 as resonance structures:

wherein throughout Formulae 2 to 5, Z represents an optionallysubstituted mono- or polycyclic aromatic ring or heterocycle whichcondenses to a nitrogen atom-containing five-membered heterocycle; whenZ does not exist in Formulae 2 to 5, a substituent similar to that for Zis bound to the position of Z; throughout Formulae 2 to 8, R₁ representsan aliphatic hydrocarbon group and R2 represent a hydrogen atom or thesame or different aliphatic hydrocarbon group as in R₁, wherein thealiphatic hydrocarbon groups may have a substituent, and in formula 2 to8, X− represents an appropriate anion.
 9. The optical recording mediumaccording to claim 8 further including at least one other organic dyecompound sensitive to visible light.
 10. The optical recording mediumaccording to claim 8 further including at least one appropriatelight-resistant improver.
 11. The optical recording medium according toclaim 8, 9, or 10 which uses a laser beam with a wavelength of 450 nm orless as a writing light.